Flame-resistant thermoplastic molding compositions and moldings made therefrom

ABSTRACT

1. A HIGH IMPACT, FLAME-RESISTANT AROMATIC POLYCARBONATE CONTAINING BY WEIGHT: (A) 2-6 PERCENT GLASS FIBERS HAVING AN AVERAGE FIBER LENGHT OF 100 TO 600UM; AND (B) 0.01 TO 1 PERCENT OF AN ALKALI METAL SALT.

l 42o rim Umted U.S. Cl. 260-37 PC 4 Claims ABSTRACT OF THE DISCLOSUREHighly flame-resistant thermoplastic molding compositions based onaromatic polycarbonates and including (1) small amounts of chlorine orbromine or alkali metal salts or nickel salts or of mixtures of chlorineor bromine and the salts and (2) small amounts of glass fibers. Themolding compositions have good processability and high impact strength.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to flame-resistant thermoplastic molding compositions and toarticles made therefrom.

DESCRIPTION OF THE PRIOR ART Various methods for reducing theflammability of aromatic carbonates from bis-hydroxy compounds areknown.

It has not proved successful to add flameproofing additives which areknown for other plastics, such as, for example, antimony trioxide, tohigh molecular thermoplastic carbonates of dihydric phenols, because theamount of such additives must be so great that the valuable propertiesof the polycarbonates are considerably impaired or the additives areincompatible with the poly carbonates or are unstable at the highprocessing temperatures of polycarbonates.

Additions of, for example, alkali or ammoniumperiiuoroalkane-sulphonates, preferably those with perfluorinated alkylradicals with about 4-8 carbon atoms, which show good compatibility withpolycarbonates and are sufficiently heat-stable (according to GermanOffenlegungsschrift 1,930,257) have proved more successful. Smalladditions of these sulphonates, in amounts of about 0.01 to about 1percent by weight, suffice to render the polycarbonates moreflame-resistant.

However, it has been found that the flame-resistant properties ofpolycarbonates containing the added alkali or ammoniumperfluoroalkane-sulphonates which have been mentioned in general do notyet suffice for practical requirements, and the grading, required formany instances of practical use, into a class of high non- [flammabilityaccording to test methods, such as, for example, the burning testaccording to Underwriters Laboratories, item 94, is not achieved.

It is particularly the dripping behavior of a chemical material onexposure to flames which is important in as much as in case of firethermoplastic compositions which burn With difficulty but drip whileburning, ignite substances which burn easily and can thus become thecause .of greater damage. However, it has not yet been possible toimprove the dripping behavior of non-reinforced polycarbonates by theadditions of sulfonates which have been described.

A similar situation applies when using additions of nickel compounds (inaccordance with German Offenlegungsschrift 1 5 s t; t

3,845,007 Patented Oct. 29, 1974 Flame-resistant polycarbonates arehitherto preferentially manufactured in industry by incorporatinghalogencontaining divalent phenols, for example tetrachloroortetrabromo-bisphenols into the polymeric molecule. The high halogencontent which is required for grading in a category of high fireresistance, for example SE 1 according to Underwriters Laboratories,item 94, can, however, have adverse effects. (Compare US. Pat.3,334,154). This is because the high halogen content causes a worseningof the processability of the polycarbonates in question, which manifestsitself in diminished flow properties and reduced heat stability of thesepolycarbonates.

Glass fiber-reinforced aromatic polycarbonates of bis hydroxy compounds,with a glass fiber content of 20-30 percent by weight, have proven to beof value in practical use, since the glass fiber reinforcement producesan improvement, relative to the non-reinforced polycarbonate, in aseries of importantproperties such as, for example, the modulus ofelasticity and the flexural strength.

While in most other thermoplastic materials such as, for example,polyamides, the glass fiber reinforcement reduces their flameresistance, the flame-resistant properties of polycarbonates areimproved by adding preferably 20-30 percent by weight of glass fibers.

Admittedly, the addition of glass fibers, in the amounts mentioned, toaromatic polycarbonates causes an em-= brittlement of the thermoplasticmolding composition and a substantial reduction in the impact strengthas compared to the non-reinforced material. Hence glass fiber additionsof 20-30 percent by weight, such as are used for reinforcing aromaticpolycarbonates from bis-hydroxy compounds and for manufacturingdimensionally stiff thermoplastic molding compositions, are unsuitablefor the manufacture of flameproof polycarbonates having a high impactstrength. On the other hand, glass fiber con tents of less than 10percent by weight when used as addi tives to aromatic polycarbonates donot produce any improvements in the fire resistance.

SUMMARY OF THE INVENTION It has now been found, in accordance with thisinven-= tion, that the high flame-resistant categories desired ofpolycarbonate molding composition, such as the SE 1 and SE 0classifications of Underwriters Laboratories, item 94, are achievedwithout appreciable effect on the processability or impact strength ofthe compositions by incorporating into the compositions (1) smallamounts of chyring or bromine or of alkali metzg galts or nickel saltsor ofb atli thhaliigeii and the salts and (2) small amounts of glassfibers.

The dramatic effect of the combination of glass fibers with halogen andor with salts is surprising because the amount of halogen or salts is,alone, generally not sufficient, in any case less sufficient to producean SE 1 or SE 0 flame-resistant polycarbonate composition and the smallamount of glass fibers would not be expected to improve theflame-resistant properties of the polycarbonate compositions. Y

DETAILED DESCRIPTION The flame-resistant thermoplastic moldingcompositions and articles provided according to this invention comprise,with the percentages given being by weight based on the total weight ofthe molding compositions:

(a) from about 93 to 98 percent of an aromatic polycarbonate; I

(b) an effective amount of (i) up to about 3 percent of chlorine orbromine, or (ii) up to about 1 percent of alkali metal salts or nickelsalts preferably those salts soluble in the aromatic polycarbonate, or(iii) mixtures of (i) and (ii); and

The chlorine or bromine is chemically bonded to an aromatic carbon atomand may be incorporated into the flame-resistant polycarbonate moldingcompositions in the form of halogen-containing aromatic polycarbonatesbased on tetrachloro or tetrabromo-bisphenol A-polycarbonates. Thepolycarbonates may be halogen-containing copolycarbonates alone orhalogen-containing polycarbonate homopolymers or copolymers mixed withhalogen-free polycarbonates.

The chlorine or bromine is incorporated into the polycarbonate moldingcomposition in amounts up to 3 percent by Weight, and preferably 1.5 to2.5 percent by weight, but if no flame-proofing additive is present, the

chlorine or bromine should be at least 1 percent by weight of themolding composition.

The alkali metal salts or nickel salts are incorporated into thepolycarbonate molding compositions in amounts up to 1 percent by weight,and preferably 0.05-0.2 percent but if no halogen is present, theflame-proofing additive should be at least 0.01 percent by weight of themolding composition.

It should be noted that the amount of (1) chlorine or bromine and (2)the alkali metal salt or nickel salt used when both (1) and (2) arepresent may be less than the amount required for each when one of themis not present in the composition. The amount of (1) and (2) needed in amixture of (1) and (2) is that which will produce an SE 2 (explainedbelow) polycarbonate material when no glass fiber is present in thecomposition.

The glass fibers are incorporated into the polycarbonate moldingcompositions in amounts of from about 2 to 6% by weight and, preferably,about 4% by weight. The glass fibers useful in the invention haveaverage fiber lengths of 100 to 600 m. and, preferably, 200-400 am.

A particularly surprising fact is that even low viscosity polycarbonate,which flows easily and which tends to drip to an increased extent, canbe employed for the manufacture of the thermoplastic molding compositionaccording to the invention, with the additives according to theinvention reducing the smoldering time and also preventing theundesirable dripping.

Because of the low glass fiber content, the high impact strength whichis demanded in many cases of practical use and is valued in aromaticpolycarbonates from bisphenol A is not worsened to any significantextent.

The small amount of salts, (fiameproofing additives), and the smalladded amounts of halogen also do not cause any decrease in the impactstrength which high impact strength is characteristic of polycarbonate.

The processability of the thermoplastic molding composition ofpolycarbonate is likewise not impaired to any I significant extent bythe small halogen content and glass fiber content according to theinvention.

Polycarbonates that may be used in the composition of the invention,include those derived from aromatic bis hydroxy compounds, and inparticular, for example, those manufactured from dihydric phenols, suchas resorcinol, hydroquinone or dihydroxydiphenyl, frombis-(hydroxyphenyl)-alkanes, such as, for example,bis-(4hydroxyphenyl)-propane-2,2, from trinuclear bis-phenols such asa,ot'-bis-(4-hydroxyphenyl)-p diisopropylbenzene, from halogenatedbis-(hydroxyphenyl)-all anes, such as, for example,4,4-dihydroxy-3,5,3,5' tetrachloro phenylpropane-2,2 or 4,4-dihydroxy3,5,3',5' tetrabromo-phenylpropane-2,2, bis (hydroxyphenyl)cycloalkanes, -sulphones, -sulphoxides, -ethers and -sulphidcs,optionally mixed with glycols, with derivatives of carbonic acid, forexample its diesters or dihalides, optionally with the conjoint use ofminor amounts of dicarboxylic acids or their derivatives which aresuitable for the formation of an ester, and which possess an averagemolecular weight of about 10,000 to 100,000, preferably between about20,000 and 50,000.

Suitable halogen-containing aromatic polycarbonates are, for example,copolycarbonates based on tetrachloroor tetrabromo-bisphenol-A andbisphenol-A or mixtures of halogen-free and halogen-containingpolycarbonate or co polycarbonate.

Possible glass fiber materials are all commercially available types ofglass fibers, such as, for example, ground short glass fibers androvings, but especially staple glass fiber, provided that they possess afiber finish which is compatible with polycarbonate.

The additives to be used are the compounds which are known and suitablefor the fiameproofing of polycarbonate. Also useful are synergisticallyacting substances.

Examples of suitable compounds are alkali salts having an organicmoiety, particularly those which are soluble in polycarbonate, and moreparticularly soluble alkali metal salts of organic carboxylic acids,preferably containing from 1 to 18 carbon atoms, and ofperfluoroalkanesulfonic acid. Examples of these salts include potassium2- ethyl-hexanate, sodium Z-ethyl-hexanate, lithium Z-ethylhexanate,potassium perfluorooctanate, sodium perfluoro octanate, lithiumperfluorooctanate, potassium salts of 5-ethyl-dioxan-1,3-yl-(5)-carboxylic acid, rubidium 2-ethylhexanate,rubidium perfluorooctanate and potassium perfluoromethanesulfonate,potassium perfl'uorooctanesulfonate and potassiumperfluorobutanesulfonate. Further examples of suitable salts are alkalisalts of lauric acid, stearic acid, oleic acid, phthalic acid monobenzylester, adipic acid monobutyl ester, p-octaylbenzoic acid, p-tert.-butyl-benzoic acid, 3-(3,5-di-tert.-butyl-4-hydroxyphenyl)- propionicacid and diglycollic acid mono-decyl ester.

Additionally, soluble nickel salts, for example the soluble nickel saltsof Z-ethyl-hexanoic acid, of lauric acid and of stearic acid may beused.

The glass fiber materials, like the fiameproofing addi tives andhalogen-containing polycarbonates described, can be added to thepolycarbonates in various ways. They can be added to the reactionmixtures before, during or after the manufacture of the polycarbonates.They can furthermore be added to polycarbonate solutions orpolycarbonate melts. Finally, they can be mixed with polycarbonategranules and these mixtures can be homogenized by subsequent meltextrusion.

In the same manufacturing process it is possible to add to thepolycarbonate, in addition to the additives according to the invention,further additives such as, for example, the pigments and effectsubstances suitable for coloring the polycarbonate, and alsomold-release agents, stabilizers and anti-oxidants.

The surprising influence of the additive combination according to theinvention on the fire resistance of aromatic polycarbonates based onbisphenol A, while retaining the impact strength characteristic of suchpolycarbonates, is illustrated by Tables 13. The undesired reduction inthe impact strength caused by high amounts of glass fibers isrecognizable. The flame-resistant polycarbonates according to theinvention can be processed in the form of powders or granules, inaccordance with the known processing methods, such as, for example, byinjection molding or extrusion, to give moldings of the most diversekind.

Molding compositions from the flame-resistant thermoplasticpolycarbonates according to the invention, which contain the additivesaccording to the invention, are suitable for the manufacture ofimpact-resistant moldings of low flammability such as are employed, forexample, in general instrument construction, precision engineering, theelectrical industry and tele-communication, and also for the manufactureof semi-finished goods, such as, for example, films, sheets, rods andprofiles.

EXAMPLES Tables 1, 2 and 3 contain Examples 1 to 20. Examples markedwith an asterisk do not relate to compositions of the invention and areincluded for comparison only.

In Tables 1 and 2 the results of flammability tests according tostandards of Underwriters Laboratories, item 94 (UL 94), and IBM60430102, are listed.

in the UL 94 test individual specimens of the dimensions x /2 x 5 inchesare vertically clamped and exposed twice for seconds each time to a bluegas flame of mm. height, the top of the gas burner is positioned about10 mm. from the lower end of a test specimen. The specimens arepositioned 12 inches above a horizontal layer of absorbent surgicalcotton. For classifying the material in a fire class, 5 test pieces inthe injection-fresh and in the tempered state (tempering is carried outat 70 C. for 7 days) are tested.

The following criteria are decisive for classification:

SE 2: Seconds Average after-burning time 525 Maximal acceptableafter-burning time after any one flame application The material may dripoff burning. SE 1:

After-burning times correspond to those of SE 2 but material may notdrip off burning. SE 0:

Average after-burning time g5 Maximal acceptable after-burning time 10The material must not drip off burning.

In the IBM 60430102 test, test specimens measuring 120 x 10 x 4 mm. aresuspended vertically and exposed to a 2 cm. high Bunsen burner flame(without an air supply). The distance of the top of the Bunsen burner is1 cm. from the bottom of the test specimen. The length of time isdetermined for which a test specimen can be exposed to the flame asdescribed above without continuing to burn for longer than 30 secondsafter removal of the flame, and without burning particles dripping offof the specimen and igniting a wad of cotton wool placed under thespecimen. A material is classed as a Class A material if after a60-second flame application time, the material will extinguish withoutproducing flaming droplets. Class B materials are those that extinguishwithin 30 seconds after a 5-second or more flame application time butcannot withstand a 60-second flame application time without exceedingthe foregoing. Thus, for example if a material extinguishes within 30seconds for flame I ployed in Examples 1-20 is the reaction product ofhis-- phenol A and phosgene having a relative viscosity of 1.28 measuredon solutions of 0.5 g. of polycarbonate in ml. of methylene chloride at25 C. It is prepared by introducing under nitrogen atmosphere, 1825parts by weight of phosgene into a mixture of 3420 parts by weight ofbisphenol A, 70.0 parts by weight of p-tert. butylphenol, 2100 parts byweight of a 45 percent by weight aqueous sodium hydroxide solution,17,500 parts by weight of distilled water and 33,000 parts by weight ofmethylene chloride at 2425 C. During the 10th and 90th minutes of theintroduction period of phosgene further 1300 parts by weight of a 45percent by weight aqueous sodium hydroxide solution are dropped in.After the introduction of the phosgene is finished 6 parts by weight oftriethylamine are added and stirred further for an hour. Then theorganic phase and the aqueous phase are separated. The organic phase iswashed first with a 2 percent by weight aqueous phosphoric acid, secondwith a 2 percent by weight aqueous sodium hydroxide solution, thirdfurther two times with the 2 percent by weight phosphoric acid andfinally with distilled water until the solution is neutral. Thereafter7500 parts by weight of chlorobenzene is added to the methylene chloridesolution and the main proportion of methylene is distilled off. Aftercooling the remaining mixture gells and the gel is chopped to smallpieces which are dried at C. under vacuum during 48 hours.

The polycarbonate of bisphenol A and tetrachlorobisphenol A employed inExamples 1-9 has a relative viscosity of 1.27 and contains 6.5 percentchlorine. This product is prepared by treating a mixture of 2.992 partsby weight of bsiphenol A, 686 parts by weight of tetra chlorobisphenol A(2.2-bis-(3,5-dichloro-4-hydroxyphe nah-propane), 50 parts by weight ofp-tert. butylphenol, 2850 parts by weight of a 45 percent by weightaqueous sodium hydroxide solution, 17,500 parts by weight of distilledwater and 33,000 parts by weight of methylene chloride with phosgene andby working up the reaction product as described above for thehalogen-free polycar bonate. v

The polycarbonate of bisphenol A and tetrabromobisphenol A employed inExamples 10-16 has a relative viscosity of 1.293 and contains 6 percentbromine. This product is prepared by reacting 3246 parts by weightbisphenol A, 414 parts by weight of tetrabromobisphenol A, 54 parts byweight of p-tert-butyl-phenol, 4300 parts by weight of 45 percent byweight of sodium hydroxide solution with the parts by weight of theremaining compounds set forth above for the tetrachlorobisphenolA-polycarbonate; the reaction product is worked up as described abovefor the halogen-free polycarbonate of bisphenol A.

The other copolycarbonates mentioned therein are prepared accordingly.

TAB LE 1 Optimizing the fire resistance, while retaining the impactstrength of polycarbonetes based on bisphenol A, by adding chlorine inthe form of tetrachlorobisphenol A-polycarbonate and/or glass fibersBurning test in accordance with Underwriters Laboratories Inc.

No. of Average burning Impact Chlorine Glass fiber smolsamples strength1 content content dering which according to Ex in percent in percenttime in, have IBM subject CMH DIN 53 453 No by weight by weight secondsdripped Classification 60430102 cm. kpJcmfl 25 20 Notself-extinguishing..." Cl. B 4 mm./ 10 secouds Did not break.

7 0 S I 01. A 4 mun/60 seconds.. 49. 25 15 B 4 mm./15 seeonds. Did notbreak.

10 7 C B 4 mm./20 seeonds Do. 6 0 A 4 mm./60 seconds Do. 3 0 A 4 min/60seconds..- 93. 20 12 B 4 Inm./20 seconds Did not break. 11 1i: mrn./30sec0nds D0.

rum/60 seconds... Do.

1 20 flame exposures were carried out in each case.

2 10 samples were tested in each case.

*Examples marked with an asterisk do not relate to compositions of theinvention and are included for comparison only.

TABLE 2 Optimizing the fire resistance, while retaining the impactstrength of polycarbonates based on bisphenol A, by adding bromine inthe form of tetrabromobisphenol-A-polycarbonate and/r glass fibersBurning test in accordance with Underwriters Laboratories Inc.

Glass No. of Bromine fiber Average burning Impact content contentsmolsamples strength 2 in in dcring which according to percent percenttime in have IBM subject CMH DIN 53 453 Ex. No. by weight by weightseconds dripped Classification 6-0430-102 k y g 25 Notself-extinguishing..... Cl. B 4 nun/10 seconds... Did not break.

7 0 SE I... Cl. A 4 min/60 seconds... 40. 15 B 4 mm./15 seconds... Didnot break.

8 5 B 4 mm./45 seconds..- Do. 2 0 I A 4 mm./6O seconds. D0. 8 9 B 4nm1./-l0 seconds. Do 2 O A 4 n1m./60 seconds. D0.

Sec footnotes bottom of Table 1.

TABLE 3 Optimizing the fire resistance While retaining the impactstrength of polyearbonates based on bisphenol A by adding ilameproofingadditives and/or glass fibers Burning test in accordance with Seefootnotes bottom of Table 1.

EXAMPLE 21 0.1 percent by Weight of potassium pcrfiuorobutancsulfonatcand 5 percent by Weight of glass fibers of average lcngth 400 ,um. wereincorporated into a copolycondensatc of bisphenol A andtetrachlorobisphenol A containing 0.9 percent by weight of chlorine togive a molding composition. Appropriate test rods from the moldingcomposition thus obtained conform to Class 1 of the fire test accordingto Underwriters Laboratories, subject 94, and are rated Class B/4 mm./45secs. according to IBM Fire Test Subject CMH 6-0430-102. Tcst specimenscorresponding to the standard specification do not break in the impacttest according to DIN 53,453.

EXAMPLE 22 0.2 percent by weight of potassium isooctanate and 6 percentby weight of glass fibers of average length 110 m, are mixed into thepolycarbonate of bisphenol A to give a molding composition. Appropriatetest specimens of the molding composition thus obtained conform to Class1 of the fife test according to Underwriters Laboratories, subject 94,and are rated Class B/4 mm./ secs. according to IBM Fire Test SubjectCMH 6-0430-102. Some of the test specimens break in the impact testaccording to DIN 53,453 while the rest do not break,

EXAMPLE 23 0.5 percent by weight of nickel lauratc and 4 percent byweight of glass fibers of average length 400 m. are mixed into thepolycarbonate from bisphenol A to give a molding composition.Appropriate test specimens of the molding composition thus obtainedconform to Class I of the tire test according to UnderwritersLaboratories, and are rated Class B/4 nun/ secs. according to IBM,Subject CMH 6O430-l02. Test specimens corresponding to the standardspecification do not break in the impact test according to DIN 53,453.

EXAMPLE 24 2 percent by weight of glass fibers of average length 400 in.are mixed into a copolycondcnsate of bisphenol A, tetrachlorobisphenol Aand dihydroxydiphenylsulfone (containing 2.25 percent by weight ofchlorine and 0.2 percent by weight of sulfur and prepared by knownmethods.

Appropriate test specimens of the molding composition thus obtainedconform to Class 0 of the fire test according to UnderwritersLaboratories, subject 94, and are rated Class A/4 mn1./ secs. accordingto IBM, Subject CMH 6-0430-102. Test specimens corresponding to thestandard specification do not break in the impact test according to DIN53,453.

EXAMPLE 25 0.1 percent by weight of sodium pcrfiuorooctanate and 4percent of glass fibers of average length 400 m. are mixed into thepolycarbonate from bisphenol A to give a molding composition.Appropriate test specimens of the molding composition thus obtainedconform to Class 1 of the fire test according to UnderwritersLaboratories, subject 94, and are rated Class B/4 mm./40 secs. accordingto IBM, Subject CMH 6-0430-102. Test specimens corresponding to thestandard specification do not break in the impact test according to DIN53,453.

EXAMPLE 26 0.05 percent by weight of potassium perfluorooctancsulfonateand 5 percent by weight of glass fibers of average length 550 m. areadded to a polycarbonate from bisphenol A to give a molding composition.Appropriate test specimens of the molding composition thus obtainedconform to Class 1 of the fire test according to UnderwritersLaboratorics, subject 94, and are rated Class B/4 mm./40 secs. accordingto IBM, Subject CMH 60430-l02. Test specimens according to the standardspecification do not break in the compact test according to DIN 53,453.

Thus from the examples it is seen that the molding composition of theinvention exhibit unexpected flameretardant or flame-resistantproperties but still retain a high impact strength and goodproccssability.

It is to be understood that any of the components and conditionsmentioned as suitable herein can be substituted for its counterpart inthe foregoing examples and that although the invention has beendescribed in considerable detail in the foregoing, such detail is solelyfor the pur pose of illustration. Variations can be made in the invention by those skilled in the art without departing from the spiritand scope of the invention except as is set forth in the claims,

What is claimed is:

1. A high impact, flame-resistant aromatic polycarbon= ate containing byweight:

(a) 2-6 percent glass fibers having an average fiber length of 100 to600 pm; and

(b) 0.01 to 1 percent of an alkali metal salt.

2. The aromatic polycarbonate of claim 1 wherein said alkali metal saltis soluble in said aromatic polycarbonate.

3. The aromatic polycarbonate of claim 1 wherein said alkali metal saltis present in an amount of from 0.05 to 0.2 percent.

4. Articles molded from the composition of claim 1.

References Cited UNITED STATES PATENTS 3,488,317 1/1970 Hechelhammer etal.

10 3,043,800 7/1962 Schnell et al 26047 3,062,781 11/1962 Bottenbruch etal 26047 3,112,292 11/1963 Bottenbruch et a1 26047 3,578,634 5/ 1971Bialous et al 26047 FOREIGN PATENTS 857,430 12/1960 Great Britain.

OTHER REFERENCES Lyons, The Chemistry and Uses of Fire Retardants(Wiley-Interscience 1970) pp. 420-1, 439.

Hattori et al. The Effects of Fiber Glass Reinforcement on theFlammability Properties of Thermoplastics (Plastics Design andProcessing, August 1967) pp. 28-30.

ALLAN LIEBERMAN, Primary Examiner T. DE BENEDICTIS, Sm, AssistantExaminer US. Cl. X.R.

10615 FP; 26041 AG, 47

1. A HIGH IMPACT, FLAME-RESISTANT AROMATIC POLYCARBONATE CONTAINING BYWEIGHT: (A) 2-6 PERCENT GLASS FIBERS HAVING AN AVERAGE FIBER LENGHT OF100 TO 600UM; AND (B) 0.01 TO 1 PERCENT OF AN ALKALI METAL SALT.